CROSS REFERENCE TO RELATED APPLICATIONS
Cross Reference is made to commonly assigned co-pending patent application Ser. No. 08/038,779 filed Mar. 29, 1993 entitled "Grid Array Masking Tape Process" the teachings of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to methods of and apparatus for immobilizing semiconductor wafers during the sawing thereof, and, more particularly, to methods of and apparatus for immobilizing (a) semiconductor wafers during the sawing thereof and (b) chips resulting from such sawing, which method and apparatus eliminate the need for a support tape and permit the wafers, qua wafers, to be sawed into chips, cleaned, undercut by plasma etching and probed, without the use of tape.
2. Prior Art
Numerous processes are known for producing plural arrays of active sites in and on a first surface of a semiconductor wafer. Each active site may comprise one or more transistors and may include an integrated circuit having other circuit components. The wafer is ultimately separated into a plurality of individual chips, also known as dies or bars, each of which includes one of the active site arrays, the array having a "top" surface comprising a portion of what was formerly the wafer's first surface.
The separation of the wafer into individual chips is effected by an operation which may be referred to as "sawing." Sawing separates the wafer along saw lines or saw paths, which may be referred to as "streets," extending between locations whereat adjacent active site arrays reside or will ultimately reside. The streets may be defined before sawing by scribing the wafer with an appropriate instrument.
Sawing, which often involves mechanical cutting, abrasion and/or erosion of the wafer, has been achieved by a number of techniques, including those which utilize rotating saw blades and vibrating tips. Properly sawing a wafer requires that the streets be accurately delineated and followed by the sawing instrumentality. This accuracy, in turn, requires that the wafer be accurately held or immobilized during sawing so that the wafer and the saw are and remain accurately positionally related during sawing.
The act of sawing the wafer produces substantial debris which includes small pieces of the wafer and possibly small pieces of the saw blade or vibrating tip. Sawing is also typically effected with the use of cooling/lubricating fluids and other substances which prevent the saw blade or tip from damaging the wafer and which prolong the life of the saw blade or tip. Fluids used in sawing wafers may significantly contribute to damage to the active sites, and, in some cases, may actually carry debris to active-site-harmful locations to which the debris might not otherwise migrate.
The debris resulting from and the substances used in sawing can degrade the performance of or render inoperative the active sites. As a consequence, wafers may be sawed into chips before carrying out the processing which produces the active sites. After sawing, the resulting chips may be maintained in a matrix. The matrix of chips is processed to produce the active site arrays on each one thereof. Often the chips are maintained in the matrix by tape-like materials adhered to the chips for that purpose.
Where active sites are first produced on wafers which are thereafter sawed into chips, tape similar to that described immediately above may be used to maintain the active-site-containing chips in a matrix thereof corresponding or congruent to the original array of active sites formed on and in the wafer. However, it may be undesirable to leave the tape in place during certain operations such as post-site-formation plasma etching procedures following cleaning of the chips to remove sawing debris. These operations may be inhibited or prevented by the presence of the tape. For example, plasma etching may cause the tape to exude gases or other substances which interfere therewith or which damage the active sites.
Nonetheless, it is desirable for the original array of active sites to be maintained and preserved in the matrix of chips during cleaning, plasma etching and probing/testing. The unsuitability of tape during some operations has led to small quantities of chips being manipulated during post-sawing operations rather than these operations being performed on the entire site array or chip matrix.
If the active sites include a micromechanical device, including a spatial light modulator ("SLM"), such as that known as a deflectable mirror device or a digital micromirror device (collectively "DMD"), each active site may be even more sensitive to the effects of the debris and fluids resulting from and used in sawing. Moreover, it is advantageous if simultaneous plasma etching of each DMD on a wafer is effected.
A DMD is a multilayered structure formed on a wafer, which includes a light-reflective mirror. The mirror is associated with an active site and is supported by a beam (cantilever, torsion, flexure, etc.) so as to be deflectable or movable between a normal position and other positions. Deflection of the mirror may be achieved by electrostatically attracting it toward (or to) a spaced electrode which is at a different electrical potential from that of the mirror. Deflection of the mirror stores energy in its supporting beam, which stored energy tends to return the mirror to its normal position. Movement of the mirror, which may be binary or analog, is controlled by the associated circuit components of the active site functioning as an addressing circuit. Deflection of the mirror is facilitated by an undercut well which underlies the beam and into which the mirror moves when it is deflected. The well may be formed by the above-noted plasma etching of one of the layers of material deposited on the wafer.
Because a DMD includes circuit components as well as a microminiature deflectable mirror, it is especially sensitive to debris resulting from sawing the wafer and to the fluids and other substances used to facilitate sawing. Specifically, such debris can enter the undercut, plasma-etched well and prevent deflection of the beam. Because the undercut well offers an attractive resting place for sawing debris, production of DMD's often starts with formation of the electrical components of the active sites, immediately followed by sawing of the wafer. Later the sawed wafer--now a matrix of chips--is cleaned and the undercut well is formed by plasma etching. This sequence prevents the sawing debris from entering the wells, the formation of which occurs only after sawing has occurred.
In use, an array or matrix of DMD's is arranged to receive light from a source. The received light which is incident on the mirrors is selectively reflected or not reflected onto a viewing surface depending on the position of the mirrors. Such reflected light is directed by each mirror onto the viewing surface in only one selected position, which may be the normal position or one of the other positions. In all other positions of each mirror other than the selected position, the incident, reflected light is directed in such a way that it does not fall on the viewing surface. Appropriate energization of the circuit components of the addressing circuit associated with each mirror permits the mirror-reflected light on the viewing surface to be presented as a rasterized array of pixels (as in a typical television) or as a scanning line of pixels (as in a line printer). Thus, the mirror of each active site is or acts as a pixel.
In some DMD production techniques, formation of the circuit components of the active sites and etching or other steps which define the mirrors are followed by the deposit or placement of a protective layer or cover thereon. Sawing of the wafer to separate the arrays then proceeds, the protective layer or cover preventing the sawing operation from damaging the circuit components and the etch-defined mirrors. After sawing is completed, the protective layer or cover is removed and the undercut wells are then formed under each mirror by plasma etching. Formation of the wells at this time obviates the sawing-related substances from entering the wells. In other techniques, protective layer or cover deposition or placement follows formation of the active sites, the mirrors and the wells.
As noted above, sawing of wafers before active site formation requires either that the resulting chips be accurately maintained in their original relative orientation during active site producing processing or that each chip be individually or small-batch processed to produce its array of active sites. These expedients are costly and their implementation is time-consuming.
One object of the present invention is the provision of a method of, and apparatus for, immobilizing both (a) semiconductor wafers during the sawing thereof and (b) chips resulting from such sawing, which method and apparatus eliminate the need for a support tape and permit previously partially processed wafers, qua wafers, to be sawed Into chips, cleaned, plasma etched and probed, without the use of the tape.